u-boot: Separate out from kernel-arch.bbclass

u-boot: Separate out from kernel-arch.bbclass

Move the UBOOT_ARCH usage into u-boot.inc and stop using kernel-arch.bbclass
since there are things in there which u-boot might not want.

This does mean the ARCH and UBOOT_ARCH variables are no longer in the environment
but I hopefully covered the main uboot usages of the variables.

Also document where these values are from and what they're supposed to mean
explictly, including the difference between UBOOT_ARCH and UBOOT_ARCH_DIR.
UBOOT_ARCH_DIR has issues which will need discussion with newer platforms
which use dts/upstream.

You might think we need to set ARCH in EXTRA_OEMAKE for u-boot but it turns
out it was ignoring the value and setting it breaks builds. This patch
therefore doesn't do that after confirmation from the u-boot devs:

https://lists.u-boot-project.org/pipermail/u-boot/2026-July/627045.html

This is confirmed by the UBOOT_DTB patch which added the kernel-arch inherit
which is much later than when u-boot was established.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>